1. Technical Field
The present invention relates generally to an improved structure of a brake system for an automotive vehicle which is designed to inform a vehicle operator of braking conditions when the vehicle travels on a slippery road surface such as a snowy road which easily induces wheel slippage. More particularly, the invention is directed to such a brake system which is used with an antilock brake system (ABS).
2. Background Art
In recent yeas, ABS control is in widespread use for maintaining slip ratios of wheels within a proper range for ensuring effective braking and stability of a vehicle body. A conventional ABS blocks a hydraulic source circuit operated by a brake pedal to provide braking control through a computer hydraulic brake pressure supplied from another hydraulic source to wheel cylinders. The structure of the ABS control unit is relatively complex and expensive.
U.S. Pat. No. 5,215,358, filed on Aug. 29, 1991, teaches an ABS control which switches the pressure in a pressure chamber of a booster of a brake unit between atmospheric pressure and vacuum. FIG. 14 shows the relation between brake pedal-depressing force and hydraulic brake pressure. In the drawing, a solid line indicates that a brake assist force is produced when the pressure chamber of the booster is exposed to the vacuum, while a broken line indicates that no brake assist force is produced when the pressure chamber is exposed to the atmospheric pressure. Therefore, even when the brake pedal-depressing force exerted by a driver is maintained constant between A and B levels, the ABS control is accomplished effectively by changing a supply pressure level to the pressure chamber of the booster cyclically within a range from a'-a to b'-b.
The above conventional ABS however encounters the following drawback. For example, when the brake pedal-depressing force is at the level A, the hydraulic brake pressure of at least level a is produced even though the brake assist pressure is not produced. This level of hydraulic brake pressure is too high to perform the ABS control especially when the vehicle is traveling on a low friction road surface such as a snowy road surface.